This invention relates to tomographic imaging, and more particularly to methods and apparatus for reducing aliasing artifacts in computerized tomographic imaging.
In at least one known computed tomography (CT) imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the "imaging plane". The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a "view". A "scan" of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two-dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
At least one known CT imaging system is available that combines a gantry rotation rate of 0.8 s with a data acquisition system (DAS) sampling rate of 1230 Hz. As a result, a projection sampling rate of 984 views per gantry rotation. Theoretical, experimental, and clinical investigations have shown that, from a standpoint of aliasing, this sampling rate is near a lower limit. It would be desirable to increase the scan rate to at least 0.5 s per gantry rotation to reduce motion artifacts and to reduce imaging times, but to do so would require a higher sampling rate. Hardware limitations limit maximum sampling rates, however. For example, hardware and software limitations may limit a DAS sampling rate to 1408 Hz. For a 0.5 s scan, 704 views per gantry rotation would be obtained in such a system, a 28.5% reduction compared to systems providing 984 views per gantry rotation. If proper compensation is not performed, view aliasing artifacts, such as streaks, will result. Such aliasing artifacts are known to be quite objectionable to radiologists.
Because the artifacts result from reduction of view sampling, it would seem logical to try to increase a number of views in a reconstruction with interpolation in a view direction. For example, a 2:1 view expansion of an acquired data set could be attempted to increase a number of acquired views from 704 to 1408. This approach, however, leads to significant reduction in spatial resolution, because view interpolation is typically a low-pass filtering process. Phantom experiments indicate that the degradation in spatial resolution can reach clinically unacceptable levels.
It would therefore be desirable to provide methods and apparatus for CT imaging with reduction of view aliasing artifacts without clinically unacceptable reduction in spatial resolution.
In many clinical scans, projection weighting prior to image reconstruction is necessary. For example, halfscan weighting has been implemented to shorten scan time by approximately 40%. Underscan weighting is used to suppress patient motion in axial scan protocols. On the other hand, helical weightings ("High Quality" or HQ mode, and "High Speed" or HS mode, for example) have been used to avoid artifacts resulting from constant table translation during a scan. It would therefore be desirable to provide methods and apparatus for CT imaging that provide reduced view aliasing artifacts for cases in which projection weighting prior to image reconstruction is necessary. It would also be desirable for such methods and apparatus to operate without requiring the use of conjugate samples, so that halfscan reconstruction is possible.